The present invention relates generally to an apparatus for removing a liquid from a gas stream and, more particularly, to the separation of water from a stream of high pressure steam flowing through an exhaust pipe of a high pressure steam turbine.
When a liquid is entrained in a gaseous stream, droplets of that liquid can cause severe erosion within the pipes which carry the gas at high velocities. In a steam generator system, the problem of pipe erosion is most commonly seen within the pipes which connect the high pressure turbine exhaust to a moisture separator reheater. Within these pipes, erosion is generally most pronounced in the extraction pipes that are joined to crossunder pipes just below the high pressure turbine exhaust snouts. The erosion of crossunder piping is therefore a serious concern to electrical utilities. When pipe erosion causes minor damage, it requires periodic weld repair, mostly in the form of cladding with an erosion resistant material, but, in more severe cases, patches have to be added to the outer surface of the eroded pipes.
Pipe erosion can be significantly reduced by reducing the amount of entrained moisture in the stream of high pressure steam flowing out of the exhaust snout of the turbine. If entrained moisture is removed from the exhaust steam of the turbine, two important advantages can be realized. The erosion damage to downstream piping can be significantly reduced and the efficiency of the moisture separator reheater can be improved.
When a fluid flows through a bend in a pipe, centrifugal forces tend to cause the fluid to be forced to the outside wall of the bend. However, although these centrifugal forces exist, a secondary flow of fluid occurs within pipe bends and has a significant effect on the behavior of liquid entrained within the gas stream. It is known to those skilled in the art that the stream lines of a fluid flowing through a bend in a pipe will not be parallel to the center line of that pipe. Instead, the fluid assumes a spiral path as it traverses the bend and this spiral path, when viewed in cross section, actually comprises a pair of spirals which exist side by side on opposing sides of a center line of the pipe which is parallel to the plane of the bend. These twin spirals cause the flow within the pipe bend to flow along the walls of the pipe toward the inside of the bend and then pass through the center portion of the pipe towards the outside of the bend. The direction of flow within this pair of spirals also determines the movement of an entrained liquid within the gas stream and, therefore, determines the specific locations where potential erosion can occur.
The existence of the pair of spirals within a gas stream flowing through a pipe bend and its affect on the flow of moisture within the gas stream can be used in a beneficial way to separate a portion of the moisture from a moisture-laden stream of gas. An apparatus for removing liquid from a liquid-laden gas stream which utilizes the above-described flow phenomenon is described in U.S. Pat. No. 3,320,729 which issued to Stahl on May 23, 1967. The Stahl patent disposes a perforated plate at the inner region of a pipe bend and provides conduits for removing liquid which passes into a chamber described by the perforated plate. It utilizes the spiral flow phenomenon which exists when a stream of gas flows through a pipe bend in such a way so as to deposit a significant quantity of its moisture along the inner radius of the bend.
It has been determined that, in a high pressure steam turbine, the flow of steam within the steam turbine is directed along the path which is somewhat analogous to that of gas flowing through a pipe bend. The turning of the stream of steam within the high pressure turbine occurs prior to its passage into its exhaust snout and, eventually, through the crossunder piping which connects the high pressure turbine to a moisture separator reheater. However, the flow of steam from a high pressure turbine is much more complex than the more predictable flow of a gas through a simple pipe bend. Therefore, the precise deposition of liquid within the exhaust pipe of a high pressure turbine cannot be as easily predicted as in the case of a pipe bend. Although the inner contours of a high pressure turbine cause the steam to be turned at several locations, the turning of the stream of steam is not simple or constant as in the case of a pipe bend having a constant radius.
The present invention incorporates a tube, or cylinder, disposed within the exhaust pipe of a high pressure turbine. The cylinder is disposed in coaxial relation with the exhaust pipe and supported within the pipe in such a way that exhaust steam which leaves the high pressure turbine passes through the internal bore of the cylinder.
The cylinder, or tube, is provided with a plurality of apertures through its wall and the relative diametric sizes of the exhaust pipe and the cylinder are such that their coaxial association describes an annular chamber, or space, between them. The apertures of the cylinder permit fluid communication between the internal portion of the cylinder and the annular chamber. This annular chamber is sealed at its axial ends in order to prevent a flow of liquid from leaving the annular chamber in an axial direction and possibly being reentrained within the stream of gas passing through the cylinder. A conduit is utilized to provide a means for removing a liquid from the annular chamber through the wall of the pipe.
In order to minimize the possibility of disadvantageous flows of liquid within the annular chamber, a preferred embodiment of the present invention includes at least two barriers connected between the cylinder and the pipe within the region of the annular chamber. These barriers extend in a direction which is generally parallel to the center line of the pipe and divide the annular chamber into at least two distinct arcuate spaces. The barriers prevent a liquid from passing from one of these spaces into the other. Each of the spaces within the annular chamber is provided with a conduit for removing its collected liquid through the wall of the pipe.
As the exhaust steam from a high pressure turbine element passes through the internal portion of the cylinder, which is disposed in coaxial relation with the high pressure steam turbine's exhaust pipe, any secondary flow spirals which exist within the cylinder will cause the moisture which is entrained within the gas stream to flow along the walls of the cylinder and pass through the apertures. After passing through the apertures, the liquid then is collected within the arcuate spaces of the annular chamber and can then be removed from the annular chamber through the conduits which pass through the walls of the pipe. After this moisture is removed, the remaining portion of the gas stream is allowed to continue through the cylinder and, eventually, through the crossunder piping which connects the high pressure steam turbine exhaust snout with the moisture separator reheater. Since the gas stream passing through the crossunder piping has had a portion of its moisture removed by the present invention, potential erosion damage to downstream piping components will be significantly reduced and the efficiency of the moisture separator reheater will be enhanced.